This invention relates generally to voltage-sensing and protective components used in conjunction with a battery-powered system, and more particularly to a way to increase the environmental resistance of a surface-mounted voltage-sensing fuse that is used as part of a voltage monitoring and protection circuit for multiple battery cells that are formed into a larger battery assembly such as that used for automotive propulsion.
Lithium-ion and related batteries are being used in transportation applications as a way to supplement, in the case of hybrid electric vehicles (HEVs), or supplant, in the case of purely electric vehicles (EVs), conventional internal combustion engines (ICEs). The ability to passively store energy from stationary and portable sources, as well as from recaptured kinetic energy provided by the vehicle and its components, makes such batteries ideal to serve as part of a propulsion system for cars, trucks, buses, motorcycles and related vehicular platforms. In one form suitable for automotive applications, individual battery cells are combined into larger assemblies such that the current or voltage is increased to generate the desired power output. In the present context, larger module and pack assemblies are made up of one or more cells joined in series, parallel or both, and include additional structure to ensure proper installation into the vehicle. Although the term “battery pack” is used herein to discuss a substantially complete battery assembly for use in propulsive power applications, it will be understood by those skilled in the art that related terms—such as “battery unit” or the like—may also be used to describe such an assembly, and that either term may be used interchangeably without a loss in such understanding.
It is desirable as part of the electrical connectivity between the various individual cells within the battery pack, as well as between the battery pack and the electrical loads discussed above, to include voltage-sensing circuitry to allow for monitoring and the related detection of abnormal voltage conditions within the pack and various battery cells. In one form used by the Assignee of the present invention, such circuitry may be connected to measurement electronics that form a part of the vehicle's low-voltage electrical system. To perform its voltage-sensing function, such circuitry further includes fail-safe components that act as a current bridge between the high voltage battery pack and the low voltage peripheral systems within the vehicle; one preferred fail-safe component used to act as a circuit-breaker is in the form of a fuse that is made up of a filament, a ceramic body surrounding the filament to protect it during normal operation and contain it during an interrupt event, and end caps to make the connection to the circuit. In a conventional form, such a fuse is an “off-the-shelf” component which is surface-mounted (such as through reflow soldering or the like) to pads formed on a circuit board or related element that connects the individual battery cells (or small groups of such individual cells) to monitoring electronics via sense lines. Because such fuses contribute resistance to the voltage sensing circuit, any variations in fuse manufacturing lead to errors in operational consistency. This lack of consistency, as well as the need to overdesign the fuses to protect the circuitry from maximum pack voltage, leads to the use of an excessively large (i.e., high voltage-rated) fuse. This in turn necessitate that a large gap between the circuit pads be used to prevent arcing around the fuse; because the circuits are already packed fairly tightly, any such increase in fuse outer dimensions makes the circuit designer's task even more complex.
Moreover, the harsh operating environment to which vehicular fuses are exposed means that they should be encased in a protective layer as a way to isolate them from the effects of such environments. Humidity in particular tends to be disruptive of a conventional fuse's electrical function; this disruption is exacerbated at the elevated temperatures that an automotive fuse may expect to encounter. Likewise, the presence of battery pack coolant or other chemical agents may also contribute to the harsh environment. Furthermore, because battery packs used in vehicular platforms operate predominantly in a dynamic (i.e., non-stationary) environment, vibration and other motion-related activities may tend to liberate such coolant onto the exposed voltage-sensing circuits, thereby compounding an already difficult operating environment. Regardless of whether from humidity or spilled coolant (or related automotive fluids), the presence of these agents may contribute to one or both of undesirably poor electrical performance and shortened life within the voltage-sensing circuit.
An even more significant difficulty arises out of the operation of a properly-functioning fuse. In particular, it is designed to interrupt the current flow of the circuit being protected during a short circuit (such as that due to an overload condition or the like) by vaporization of the filament. During the fusing event, there is a dramatic temperature rise within the fuse up to the melting (i.e., vaporization) point of the filament such that electrical arcing occurs until enough of the filament is consumed to break the circuit and interrupt the current flow entirely. Any air surrounding the filament becomes superheated. Significantly, if the overload condition exceeds what the fuse is rated for, the body and end caps may not contain the arcing; this in turn leads to the formation of superheated gases which may propagate to other nearby components (such as adjacent voltage-sensing or related fusing circuits). While the destructive effects of such a fusing event may be halted or minimized by the use of specialized fuses (such as those rated for automotive high voltage battery packs), this entails large packaging requirements and high costs. Moreover, to the extent that such a conventional fuse may use a thin overcoating, it cannot act as a seal to shield the fuse from the local environment associated with a high voltage battery pack. The present inventors have determined that the portion of the overcoating adjacent the corners of the fuse is particularly susceptible to being breached under these harsh environmental conditions.
One particularly destructive attribute of a breech (either with or without the overcoating discussed above) is the possibility of dendritic growth (and concomitant corruption of other, previously-unaffected circuits) when exposed to the environment. For example, upon activation of the fuse as a circuit breaker in response to a high voltage (about 400V and above, for instance) short circuit, the present inventors have determined that the violent fusing may either burn a hole through the circuit's substrate material or spread out over the substrate surface, and that this has a possibility to cover the nearby area with conductive carbon that through subsequent dendritic growth into adjacent circuits can lead to other short-circuiting events. The present inventors have determined that such dendritic path formation and growth is particularly likely to form from two methods, including (a) repeated battery heating and cooling that leads to condensation (which includes both water and various conductive contaminants) inside the battery assembly, and (b) coolant leaks that arise out of various types of failure events. This dendritic formation is especially problematic in the presence of ionic aqueous deposits (such as from coolant or the like which, like the water mentioned above, may evaporate to leave conductive contaminants behind that can build up and provide the resistive short-circuit). As such, dendritic growth can occur at any point where the sensing circuit is not sealed against such an environment.
As such, what is needed is an encapsulating material that can provide protection of the fuse against its ambient environment while operating and performing its primary function in the circuit during a high voltage short circuit event in chemically harsh environments as a way to ensure the continued circuit protection. Such encapsulating material would enable the use of conventional surface-mount fuses with closely-tailored voltage ratings, which in turn allows for smaller packaging size and lower-cost materials.